Telephonic device designs, and their accompanying microphone assemblies designs, generally address background noise and reverberations that interfere with the sound quality a speech signal received by the handset user. Such background noise and reverberations diminish speech intelligibility perceived by the far-end party on the telephone call. In the prior art, acoustical noise-canceling (also referred to as “close-talking”) microphones are typically used to address this noise. Such microphones are used in handsets and boom headsets, for example.
Microphones are also susceptible to “puff” or “plosive” noise, resulting from plosive sounds in the talker's speech itself. Plosive sounds result from speaking words which require a substantial amount of air in order to articulate the sound. Examples of plosive sounds are the constants “p”, “t”, and “d”. Such plosive sounds result in “puff” or “plosive” noise when the air hits the microphone assembly, causing detectable turbulence. For example, the spoken words, “Peter Piper Picked a Peck of Pickled Peppers” contain the plosive “p” sounds.
In the prior art, telecommunications handsets and hands-free devices such as boom headsets have advantageously placed the microphone ports in the mouthpiece close to a talker's lips. The distance between the microphone ports and the talker's lips is often referred to as the “modal distance”, and is typically between 12 and 60 mm.
Minimizing the modal distance provides for increased pickup of the talker's speech signal, which decreases as the modal distance increases. Thus, it is generally desirable to minimize the modal distance regardless of the telecommunications application (e.g., handset or hands-free device) utilizing the microphone. In this manner, the acoustical signal-to-noise ratio, and thus sound quality, is maximized for each application.
However, background noise and reverberation pickup by the microphone are not affected by modal distance. In noisy locations, such as encountered in commercial work spaces like a stock exchange floor or in a trade show, acoustical noise-canceling microphones are utilized to reduce the effects of background noise. Noise canceling microphones are more sensitive to the near-field spherical acoustical signals radiating from the lips than the far-field noise plane waves approaching the microphone's ports. Furthermore, noise canceling microphones discriminate against incoming random-incident background noise in favor of the talker's speech by pointing its pickup sensitivity spatial lobe preferentially toward the talker's lips.
Noise-canceling microphone assemblies use multiple acoustic inputs, typically two ports on the housing surface sampling the sound, with each port leading to one side of a gradient electret microphone element diaphragm. Improved noise cancellation performance is achieved with decreased modal distance. Gradient microphone elements, while performing the desired noise-canceling function on background noise versus the talker's speech, unfortunately have an elevated sensitivity to puff signals relative to omni-directional (one port) microphone designs, which cannot cancel noise. Sensitivity to puff noise results from turbulent audible noise caused by the puff signals at each of the two inlet ports on the housing, with the resulting turbulences uncorrelated at the two ports and random in phase.
In the prior art, several attempts have been made to reduce the effects of microphone puff noise. For example, in one typical solution, foam windscreens are placed around the microphone. However, use of such foam screens increase the distance between the microphone and the user's lips, reducing the effectiveness of noise canceling microphones.
Thus, improved designs for high noise-canceling microphone assemblies with low “puff” pickup are needed. In particular, there is a need for improved microphone assemblies are that enable noise cancellation, minimize the effects of puff noise, and provide good speech signal pickup.